With electrosurgical devices, in particular those used in monopolar high frequency (HF) surgery, an electromedical device is used to supply power to an instrument to be used by the surgeon for desired operations such as for example, tissue fusions, coagulations, ablations or the like. If the instrument is monopolar, it is supplied with HF power via a line. A neutral electrode, to be attached to the patient over a large area, is used to close the electrical circuit. A current path is therefore provided that extends from the active instrument via the body of the patient back to the electromedical device. The current should pass through the neutral electrode with as little damage to the tissue as possible. Any rises in temperature at the neutral electrode should remain small to avoid thermal tissue effects or even damage at this point. It is therefore desirable to limit the current density. For this reason, the neutral electrode has a large contact area. The greater the contact area of the neutral electrode, the lower the resultant current density and the heating it induces.
If electrodes have a large surface area, they must contact the patient as fully as possible to prevent local concentrations of current. The electrode is therefore generally split into two sections that both contact the skin of the patient. The impedance to be measured between the two sections, or the contact resistance, is compared to a limit value. If the impedance exceeds the limit value, then activation of the HF surgical device is prevented.
The limit value for the tissue impedance must be set to a relatively low value to effectively detect an incorrect (i.e., incomplete) contact of the neutral electrode. However, in patients with dry skin or a thick layer of subcutaneous adipose tissue, this will lead to the impedance threshold being exceeded and the deactivation of the HF surgical device even though the neutral electrode is correctly attached to the patient. This leads to difficulties in the operating theatre. The usual approaches of cleaning or wetting the skin, using larger neutral electrodes or searching for more suitable points at which to attach the electrodes so that the value falls below the limit value results at least in a disruption of the usual surgical routine and also to an additional time expenditure. An increase in the threshold value is not a real alternative because of the associated danger of failing to recognize an incorrect attachment of the electrode to patients with a low skin resistance. This could result in damage to the skin at the point of attachment.